Impaired intestinal barriers are associated with infection. Vitamin D and its receptor (VDR) levels are inversely related to chronic inflammation in infectious diseases. Salmonella enterica serotypes are invasive enteric pathogens spread through fecal contamination of food and water sources, and represent a constant public health threat in US and around the world. The objective of this application is to study VDR regulation of intestinal tight junctions (TJs) in response to Salmonella infection. Salmonella induces the disruption of TJs during infection. This is associated with a decrease in trans-epithelial resistance, increase in tracer permeability, and TJ protein alterations. Our publications and preliminary data have shown that: (1) lack of VDR makes the host susceptible to Salmonella invasion; (2) Salmonella targets TJ proteins (e.g. ZO-1, Claudin) and facilitates pathogenic enteric bacterial invasion. (3) We have identified the sequence of functional vitamin D response element in Claudin 5, a potential novel target gene of VDR; (4) impaired VDR leads to reduced expression of TJ proteins, abnormal TJ structure, and increased intestinal permeability in infection; and (5) probiotics enhance VDR function and inhibit Salmonella infection. Thus, we hypothesize that: ?intestinal epithelial VDR regulation of barrier function is aberrant or lost in infectious states, and restoring VDR function will attenuate infection and chronic inflammation.? We have now developed state-of-the-art transgenic models, e.g. intestinal epithelial VDR conditional knockout (VDR?IEC) mice, conditional transgenic VDR over-expressing (Tg-VDR) mice, along with loss- and gain- function assays of VDR in organoids to investigate VDR actions that critically affect barrier functions of the intestine. Aim 1. Determine the molecular mechanism by which intestinal VDR regulates barrier function that is essential for mucosal homeostasis. We will elucidate the mechanism for abnormal epithelial TJs in the VDR-/- organoids and VDR?IEC mice. Aim 2. Determine the mechanisms by which impaired intestinal barrier is associated with severe infection in VDR?IEC mice. We will determine the physiological role and molecular mechanism of intestinal epithelial VDR, as an upstream regulator, in linking TJs in the VDR?IEC mice with Salmonella infection. Aim 3. Restore the intestinal epithelial VDR expression by force expression of VDR or probiotic treatment to alleviate infection and associated inflammation. We will determine the roles and mechanisms of A) forced intestinal epithelial VDR expression in restoring TJ integrity in Salmonella infected Tg-VDR mice; and B) enhanced intestinal VDR expression by probiotic lactic acid bacteria in infected mice. This knowledge can then be exploited to define strategies to prevent and treat pathogen infection, by restoring intestinal VDR.